This relates to semiconductor light-emitting devices.
Some modulator may have a quantum well layer and a bulk layer interposed between a p-type semiconductor layer and a n-type semiconductor layer, to modulate light passing through a layer such as the quantum well layer, by applying an electric field and changing absorptance or a refractive index. One example is known as an electric field absorption-type modulator (EA modulator), and another example is known as a Mach-Zehnder modulator (MZ modulator). Alternating current signals (AC signals) in a single ended mode are applied to the modulators (specifically, EA modulator). Recently, for improving quality of electric signals and for low-voltage drive (low amplitude drive), another mode to apply electric signals by differential signals has begun to be used. For example, JP 2012-151244 A and US 2014/0198816 A1 disclose a semiconductor light-emitting device, where an EA modulator and a semiconductor laser (light source) are integrated on the same substrate and the EA modulator is driven in a differential drive mode.
The differential drive mode for driving a modulator has problems as follows:
[Problem 1]
Two types of modulators are known: One is equipped with an electrode on one side for a p-type semiconductor layer and with another electrode on an opposite side for an n-type semiconductor layer. The other is equipped with the both electrodes for the p-type semiconductor layer and the n-type semiconductor layer on the same side. The modulator is mounted on an element mounting surface of a submount (e.g. carrier, mounting substrate) with a pair of differential transmission lines formed thereon. The type of modulators with the electrodes on both sides may have lower electrodes (mainly for n-type semiconductor layer) connected to one of a pair of differential transmission lines (e.g. cathode line) with solder or brazing material and may have upper electrodes (mainly for p-type semiconductor layer) connected to the other of the pair of differential transmission lines (e.g. anode line) with a wire.
The anode line has ideal impedance just before the wire. By contrast, the cathode line is bonded to the electrode of the modulator with the solder or brazing material, having a connection portion in a shape equal to or larger than the electrode, causing electrical reflection. The differential drive requires equality of electric signal quality between the anode line and the cathode line. The major factor of the above reflection, which is only inherent in the cathode line, downgrades the electric signals and optical waveform quality (optical signal quality) after modulation.
[Problem 2]
The type of modulators with the electrodes on both sides may have large differences in configuration from the quantum well layer to the electrode. In general, the quantum well layer has a width as narrow as some micrometers, for improving optical and electrical confinement properties. The distance from the quantum well layer to the electrode for the p-type semiconductor layer is as small as some micrometers. By contrast, the n-type semiconductor layer under the quantum well layer is as large in size as a substrate, as large as a rectangle with a side of more than 100 μm, as thick as about 100 μm.
The quantum well layer, to which the electric field is really applied, is almost thousand times farther from the electrode for the p-type semiconductor layer than from the electrode for the n-type semiconductor layer, differentiating respective impedance characteristics from the quantum well layer to the respective electrodes. In spite of applying the same electric signal (with the same impedance characteristics) from both sides, unbalanced differential signals are actually applied and the optical waveform quality deteriorates.
[Problem 3]
The modulators tend to be used with a termination resistor (matching resistance, matching resistance) positioned parallel thereto, for impedance matching (e.g. 100Ω for a differential drive) with a driver IC. Wires are used for connection from the transmission line to the electrode for the p type semiconductor and other connection to the termination resistor (U.S. Pat. No. 6,057,954). Adjusting the length of the wires can improve the high frequency response characteristics (electro-optical (E/O) characteristics, S21 characteristics) of the modulator. To achieve the effect, the termination resistor is positioned on the opposite side of the modulator from the transmission line. This arrangement makes asymmetrical the elements affecting high-frequency characteristics from each of an anode line and a cathode line constituting a pair of differential transmission lines to the termination resistor.
On the side of the p-type semiconductor layer, one of the wires is provided between the electrode of the modulator and the termination resistor. On the side of the n-type semiconductor layer, the transmission line is connected to the electrode of the modulator with solder or brazing material, having a larger width and having inductance and capacitance. This differentiates the impedance characteristics from each of the anode line and the cathode line to the termination resistor, leading to an unbalanced transmission condition of the differential signals, degrading characteristics.
Regardless of the differential signals or a single ended signal, the termination resistor is generally set to have about a 50Ω impedance for driving impedance matching. Any factor included between the termination resistor and the transmission line may create an impedance mismatch and cause a malfunction of transmitting a desired voltage, for example.
This is to aim at impedance matching.